Antenna array



E. G. HILLS ANTENNA ARRAY March 10, 1953 2 Sl-IEETS-Sl-IEET 1 Filed Feb. 16, 1949 lnvemor Elmer G. HIHS March 10, 1953 E. G. HILLS 2,631,238

ANTENNA ARRAY Filed Feb. 16, 1949 2 SHEETS-SHEET 2 Band Band Puss Pass I02 Fllter I04 Filrer I06 Inventor Elmer G. HiHS Arty.

Patented Mar. 10, 1953 ANTENNAARRAY Elmer G. Hills, Chicago, Ill., assignor to Belmont Radio Corporation, Chicago, 111., a corporation of Illinois Application February 16,1949, Serial No. 76,673

3 Claims. (CLZ 250-3353) This invention relates to television antennas, and particularly to multiple antenna arrays for television receivers.

A multiple antenna array has the advantage that instead of relying upon a single antenna to I receive the broadcasts from all the stations in a given locality, it contains several antennas which are individually adapted to receive the signals from difierent stations or groups of stations. One

commonly-used array includes two antennas, one being oriented and tuned for a high-frequency station or band and the other being oriented and tuned for a low-frequency station or band. For best results, these antennas should function independently of each other, which is to say, that for any given station or channel, only one of the antennas should furnish the signals of that station or channel to the receiver. In such commonly-used multiple antenna arrays, the antennas are connected directly to a common transmission line which leads to the receiver, so that there is always a connection to both antennas. This makes it extremely difiicult, if not impossible, to orient each antenna accurately for the optimum reception of signals in its particular channel, due to the interaction which occurs because of the reception of signals in this channel by the other antenna or antennas. Thus, the advantage of using more than one antenna is partially ofiset by these complicating factors.

An object of the present invention is to provide an improved form of connection between the common transmission line and the various antennas whereby each antenna furnishes only the signals in its particular channel to the receiver, and no other antenna is effective to furnish Signals in that particular channel to the receiver.

Another object is to provide an improved multiple filter unit having individual filter devices for passing signals from the respective antennas to the receiver, each filter device being arranged to pass only the signals in the predetermined band of frequencies which its particular antenna is efiective to pick up.

A further object is to provide novel means to prevent the loading of one antenna circuit by another antenna circuit in a multiple antenna system.

A still further object is to provide improved means for coupling an unbalanced coaxial line to a balanced antenna and to a balanced transmission line.

A feature of the invention is an arrangement of filters for coupling a common, mission line to a plurality of unbalanced coaxial 2 lines leading respectively from the various an tennas. In one embodiment of the invention, a plurality of band-pass filters are arranged with certain parts thereof in series with the common transmission line. In another embodiment, these parts are arranged in parallel with the common line.

Another feature is the cellular construction of the multiple filter unit. Each individual filter is mounted in a separate shielding cell from which it can readily be removed for repair or replacement purposes. The initial installation need not include a full complement of filters. Additional filters may be added as required, without disturbing the system.

Still another feature is the manner of coupling an unbalanced coaxial line to a balanced, folded dipole antenna without disturbing the balance of the antenna. The coaxial line is extended from the point where it connects to the dipole through a tube of the dipole and is brought out near the center of the grounded member of the dipole. 1

The foregoing and other objects, features and advantages of the invention will be better understood from a study of the following description taken in connection with the accompanying drawings, wherein:

Fig. 1 is a perspective view showing a multiple antenna array embodying the principles of the invention;

Fig. 2 is a perspective, partially schematic view showing an antenna in conjunction with the multiple filter unit;

' several constituent unit; and I Fig. 3 is an exploded perspective view showing parts of the multiple filter Fig. 4 is a schematic diagram of amodified form oi'filt'er unit which maybe employed.

In practicing the invention, an array consisting of seven television antennas is mounted upon a vertical mast, there being one antenna for each of the seven channels or stations which may be operated in a given television broadcasting area.

This'number, of course, may be varied for difierent areas. Each antenna is individually designed to pick up signals in a particular frequency channel, and it is individually oriented on the mast for the most favorable reception of signals broadcast by the station which operates in that particular channel. Coaxial lines are coupled to the various antennas without disturbing the balance of the antennas, and these lines lead respectively to the individual filter devices in a balanced transmultiple filter unit; Each filter device is; arranged to pass only the frequencies in the channel which its particular antenna is designed to pick up. A common transmission line leading to the receiver is coupled to the multiple filter unit. This line receives from each antenna only the signals which that antenna is designed to receive, the filter unit functioning to exclude all other signals which inadvertently may have been picked upby that antenna. In this way, each antennaisrprevented from loading any of the other antennas.

Referring now to Fig. 1, there is illustrated a multiple antenna array consisting of seven television antennas IU, l2, l4, l6, I8, 20 and 22. These antennas are of similar construction,.each comprising a folded dipole 24, a pair of parasitic directors 26 and a parasitic refiector'28. Each antenna is designed to pick up signals in a certain frequency channel assigned to a particularstation. The antennas are mounted in vertically spaced relation on a metallic mast 30. A metallic support rod 32 in each antenna structure is securedto the mast 39 .by individual clamping ..means..% or the like, so that each antenna can be individually oriented for the most favorable reception of signals in its corresponding frequency channel. The directors 25, reflector 28 and dipole 24 of each antenna are secured to the support rod 32 by suitable connectors as indicated.

Leading from the dipole '24 of each antenna is acoaxial line orcable 36. As shown best in Fig. .2, each cable 3-6 is coupled to its dipole 24 in such ama-nner as to prevent the coaxial line 36 (which is unbalanced with respect to ground) from disturbing the balance of the dipole 24. Each of the folded dipoles 24 comprises a length of bent tubing having a lower member 38 and two upper members 40, with a gap between the ends of the members All. The lower member 38 is grounded through the connector 42 which secures the member 38 to the support rod 32, the path to ground being established through the rod 32 and the .mast 3!]. The coaxial cable 36 has an outer conductor which is at, or near, ground potential, and aninner conductor Which is at a considerably higher potential. The outer and inner conductors of the cable 36 are connected respectively at points 44 and d6, Fig. 2, to the terminals of the dipole 24 adjacent to the gap between the members 40. The impedance between these terminals matches the impedance of the cable 36. The cable 36 then is extended through the tube on one side of the dipole 24 and .is brought out through anopening 48 near the grounded center part of the lower member 33. By running the cable 36 through the dipole in this fashion, the unbalance which normally results when the cable is connected directly to the terminals as and 4B of the dipole is avoided in the present instance.

The various coaxial cables 36 leading from the respective antennas are connected to a multiple filter unit 50 mounted on the mast 3D, and from this filter unit 50 a common, balanced transmission line 52 leads to a television receiver 5 3, Fig. 2. The filter unit 50 comprises a plurality of individual filter devices 56, 58, 60, E2, Bd m and 68, each of which is disposed in an individual shielding cell or compartment. The mechanical construction of this filter unit will be described subsequently. Each of these filter devices has characteristics such that, for any given locality, only the signals broadcast .by a particular television station will pass from the associated antenna .to .the. receiver. 54.. vInanother-locality sucha filter device maypass signals =;frozn anotherstation "4 operating on a nearby frequency. However, two stations whose broadcast channels are so close together that their signals could pass through the same filter device would not be permitted in the same television broadcast area.

Other functions performed by the filter devices in the filter unit 50 are to match the impedances of the individual antenna lines 36 (which are unbalanced) to the impedance of the common transmission line 52 (which is a balanced line), and also to isolate electrically the various antennas lines 35 from each other. The filters prevent the antennas from loading one another by thus eliminating any tendency toward interaction between the-antenna lines at their common junction point. The use of coaxial antenna lines Y prevents interference between the various lines which must Ice-brought down along the mast. Therefore, it becomes an easy matter to orient or otherwise adjust each antenna individually without affecting the reception from any other antenna in the array. Heretofore, the lack of independence among the various antennas has been a serious handicap in properly setting up a television receiving installation. By employing the disclosed filter unit 50, the utilization of a large number of antennas for improved quality of reception becomes practicable.

'The filter devices 56 to 6B are of identical construction except for 1 their different frequency response characteristics, and hence a detailed description of one,(for example, the filter device' 62) will sufiice for all. As shown in Fig. 2, the filter device 62 includes a transformer If! that serves to transform the impedance of the coaxial line 36 to that of the common transmission line 52. In series with the primary of the transformer 10 is a variable capacitor 1.2. Across the secondary of the transformer I0 is connected the parallel combination of a fixed capacitor 1 3 and a variable capacitor 16. The primary side of each filter device as 62 is connected across its respective coaxial line 36 leading from the associated antenna. The secondary sides of all the filter devices are connected in series to the transmission line 52. These secondary sides consist of parallel resonant circuits with high C/L ratios, each being tuned to a particular frequency channel. As to frequencies lying outside the particular channel for which the filter device is tuned, the parallel resonant circuit'has a very low impedance. Each filter device, therefore, presents ,a

very low impedance to .all frequencies except those in its particular channel. Inasmuch as the channels for which the various filter devices are tuned are mutuallyexclusive. of each other, there is no possibility that any perceptible coupling will exist between the filter devices. Hence, there is no loading of one antenna by another.

.As afurther aid in suppressing the unwanted frequencies in each antenna line, the primary side ofeach filter device is so arranged that it presents a series resonant circuit which affords a very high impedance to signals outside of the particular channel concerned. As to unwanted signals, therefore, each filter device functions on its primary side as an open circuit and on its secondary side as a short circuit. The transformers as 10 also serve to matchthe impedance of each coaxial line 36 to that ofthe common transmission line 52, as mentioned previously.

Referring now to Fig. 3, which illustrates the physical arrangement .of the multiple filterunit ,511, .a central .quillorltubular supportmember Bil- .5 secured (to. shield plates .82 whichhexterid in an approximately radial fashion therefrom. These plates divide the filter unit into as many radial compartments or cells as there are filter devices. Each filter device, such as 82, is mounted on the inner face of an outer shield plate 84 having tongues 96 at the corners thereof which are received in slots 88 in the outer portions of the plates 82. These outer portions of the plates 82 are bent so that filter-supporting plates 84 of uniform size can be employed without interference between the parts when they are installed or removed. When each filter device is mounted in its cell, it is shielded by the Walls of the cell from every other device in the unit 56. The entire unit is disposed in a round can or housing 99 for which a suitable cover or lid (not shown) is provided.

In Fig. 4 there is shown schematically an alternative arrangement of filter devices for connecting the coaxial lines 35 to the common transmission line 52. Three of these devices are illustrated herein. Coils 92, 96 and 98, capacitors 94, 9'! and 199, and band-pass filters 192, 164 and 196 serve to connect the three lines 38 in parallel to the line 52. The band-pass filters I62, E94 and 488 are of such a character that they present substantially open circuits for all frequencies except these within their respective channels. The series resonant combinations of coils as 92 and capacitors as 94 also help to suppress the unwanted frequencies.

From the foregoing it will be appreciated that I have provided a multiple antenna system in which any practical number of antennas may be operated independently of each other. The max.- mum number of antennas which would be required under existing conditions is seven. In localities where there are fewer stations, a correspondingly smaller number of antennas may be employed. Additional antennas may be added as other stations become available in that locality. The cellular construction of the filter unit 58 enables individual filter devices to be added or removed without disturbing the other connections. Still other advantages of the disclosed system not specifically mentioned above may readily occur to those skilled in the art.

While the invention has been described with reference to several preferred embodiments thereof, modifications of such embodiments may be possible without departing from the principles set forth above, and it is intended that all such modifications shall be included Within the scope of the appended claims.

I claim:

1. A filter unit for a multiple television antenna system having antennas which are individually adapted to pick up signals broadcast from various television stations, such filter unit comprising a plurality of individual filter devices for the various antennas, a shielding structure having individual open-mouthed cells for receiving said filter devices, and a housing for the filter unit, each of said filter devices including a supporting plate having provisions for attachment thereof to said shielding structure at the mouth of a particular cell therein, and also including a network of reactive elements mounted on said plate and tuned to pass a band of frequencies related to a particular television station.

2. A multiple antenna system for a television receiver comprising a plurality of individual antennas of the folded dipole type having such configurations to receive signals in diiferent frequency channels, each of said antennas including a tubular conductor having a portion near ground potential and a pair of remote terminal ends which are substantially above ground potential,- each of said tubular conductors having an opening therein near the grounded portions thereof, a respective unbalanced shielded coaxial transmission line individually associated with each of said tubular conductors and having an end extending through said opening and into said tubular conductors to one of the remote terminal ends with the conductors of said coaxial transmission line being connected to said terminals of said antenna, a multiple filter unit including a plurality of individual filter devices, each of said filter devices including primary and secondary circuits with the primary circuits being individually connected to said coaxial transmission lines, said filter devices having frequency-response characteristics such that each device is able to pass only the signals in the channel of the antenna to which it is connected, and a common transmission line of the balanced type connected to said secondary circuits of said filter devices for applying the signals to the receiver.

3. A multiple antenna system for a television receiver comprising a plurality of individual antennas of the folded dipole type having such configurations to receive signals in given frequency channels, each of said antennas including a tubular conductor having a portion near ground potential and a pair of remote terminal ends which are substantially above ground potential, each of said tubular conductors having an opening therein near the grounded portions thereof, a respective unbalanced shielded coaxial transmission line individually associated with each of said tubular conductors and having an end extending through said opening and into said tubular conductor to one of the remote terminal ends with the conductors of said transmission line being connected to said terminals, a multiple filter unit including a plurality of individual band pass filter devices, and a common transmission line of the balanced type for connection to the receiver, each of said filter devices including a transformer having a primary side connected to one of said coaxial transmission lines and a secondary side connected to said common transmission line.

ELMER G. HILLS.

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